Field of the Invention
This invention relates to a wide-field eyepiece with inside focus having a high magnification for use in a telescope, binocular or the like, and more particularly to an eyepiece for use in a monocular telescope for bird-watching.
Description of the Prior Art
In normal eyepieces, the distance between the eyepiece and the eye point tends to be shorter as the focal length of the overall system becomes smaller. This is because, since the optical system of normal eyepieces is similar to a telecentric optical system, the back focal point of the overall lens system is close to the eye point and the back focal length of the overall system becomes smaller as the focal length of the overall system becomes smaller.
For example, in an eyepiece which has a magnification of .times.20 and a focal length of 12.5 mm, the distance to the eye point is small and not larger than about 10 mm, and in an eyepiece which has a magnification of .times.33 and a focal length of 7.6 mm, the distance to the eye point is smaller and not larger than about 6 mm. Such eyepieces are difficult to use. Especially for a spectacled person, it is said that the distance to the eye point should be not smaller than 20 mm. With glasses, it is difficult to view through such an eyepiece having a small distance to the eye point.
With respect to the flatness of the image plane, the normal eyepieces have a large negative curvature of field and when an object at the center of the field is brought into focus, the image in the periphery of the field is blurred. This is because the normal eyepieces are formed by combination of lens elements or cemented lenses having positive refracting powers and the Petzval sum is a large positive power in such lenses having a positive refracting power. When the Petzval sum is a large positive power, a large negative curvature of field is generally generated.
There has been known a high magnification eyepiece which has a distance to the eye point not smaller than 1.5 times the focal length, is excellent in flatness of the image plane and is small in distortion as disclosed, for instance, in Japanese Unexamined Patent Publication No. 2(1990)-123319 and Japanese Patent Publication No. 5(1993)-67005.
When bird-watching through a high magnification monocular telescope, the bird-watcher generally wants to see the object bird in a true color. However the conventional eyepieces for the monocular telescopes or the like are not good in chromatic aberration and color reproductivity is unsatisfactory. For instance, color shading occurs in the periphery of the field.